1. Technical Field
The present invention relates to optical coherent communications systems, and more particularly to a system and a method for adjusting a constellation de-mapper in response to error feedback.
2. Description of the Related Art
In optical coherent communications systems using Pulse Amplitude Modulation (PAM), Quadrature Amplitude Modulation (QAM) or may Phase Shift Keying (m-PSK), a received signal may be corrupted by a carrier frequency-offset (CFO) due to local oscillator drift. Since a CFO causes a time-varying rotation of the received data symbols, it has to be accurately estimated and compensated for prior to symbol detection at the receiver, particularly in the case of large-size symbol constellations.
Non-data-aided (or blind) techniques for extracting symbols from a signal have been proposed in order to preserve bandwidth efficiency and simplify the transmitter architecture. A standard approach to joint blind equalization and carrier recovery consists of first performing equalization using the constant modulus algorithm (CMA), and then estimating the carrier phase and frequency from the equalized output. For m-PSK or QAM systems, frequency offset and carrier phase can be recovered using the fourth-power of the equalizer output.
In order to extract symbols from a modulated signal, a constellation de-mapper, such as the differential 8-PSK constellation de-mapper shown in FIG. 1, is used. The input 12 is the received complex signal sequence, after frequency offset and phase offset have been removed. The phases of two successive inputs 14 and 16 are compared 18 and then passed through the Phase Shift Keying threshold 20, which places the input on the constellation. The constellation de-mapper 22 produces as output 24 the binary data (bit) stream. Obviously, the constellation de-mapper has to correspond to the PAM/QAM/m-PSK signal generation processing at the transmitter to guarantee the output bits' correctness. The de-mapper is usually designed after the transmitter generates the m-PSK signal through back-to-back calibration. However, due to the limits of the blind CFO estimation algorithm, the de-mapper could change when the CFO exceeds a certain threshold. When the constellation de-mapper changes, the binary data (bits) output cannot maintain correctness. When this occurs, the CFO has caused the symbols to shift, resulting in a de-mapper mismatch.